The present invention relates generally to a method of manufacturing a high temperature superconductor and more specifically to a pulsed laser deposition method of applying a plurality of alternating thin layers of non-superconducting material and superconducting material in high temperature superconductors.
The desirability of providing efficient high temperature superconductors for operation at 20xc2x0 K and higher is well known. Indeed, there has been an enormous amount of experimental activity in these so called high temperature superconductors since research in the mid 1980s first demonstrated dramatic gains in raising the maximum critical transition temperatures from the 20xc2x0 K range to the 90xc2x0 K range.
In general, superconductors and superconducting material exhibit zero resistance when operating at temperatures below their maximum critical transition temperature. This quality of operating at zero resistance facilitates the construction and operation of highly efficient devices such as superconducting magnets, magnetic levitators, propulsion motors and magnetohydronamics, power generators, particle accelerators, microwave and infrared detectors, etc.
In addition to the quality of operating at zero resistance, superconductors display other, unusual characteristics. For example, a surprising effect of superconductivity is that magnetic flux is expelled from superconductors. This is commonly known as the Meissner or Meissner-Ochsenfeld effect. Other unexpected phenomena include current flow via electron pairs rather than individual electrons and large scale quantum behaviors such as flux quantization and flux tubes.
According to current accepted nomenclature, superconducting materials fall within two broad categories, Type I and Type II. The Type I materials are pure metallic elements. Type II materials are alloys or compounds and are characterized by their ability to retain superconductive attributes in the presence of applied magnetic fields. Unlike Type I materials, Type II materials tolerate some degree of applied magnetic flux intrusion into their interiors without destroying the superconducting state. More specifically, as applied flux and/or temperature increase, microscopic flux tubes or fluxons begin to develop within the material. In this mixed state of superconductivity, the material within the flux tubes is in a normal state of resistivity. The material surrounding the flux tubes remains in a state of superconductivity. Since the Type II materials will support mixed superconductivity at elevated temperatures and applied magnetic field intensities, the Type II materials seem to be the best candidates for, commercial application and further development.
More specifically, under equilibrium conditions, magnetic flux penetrates the; bulk of a type II superconductor above the lower critical field. Over most of the available magnetic field-temperature (H-T) space, H greater than Hc1, this magnetic flux exists as a lattice of quantized line vortices or fluxons. Each fluxon is a tube in which superconducting screening currents circulate around a small non-superconducting core. Bulk superconductivity is destroyed when the normal cores overlap at the upper critical field. In isotropic materials such as Nbxe2x80x94Ti and Nb3Sn, vortex lines are continuous, but the weak superconductivity of the blocking layers of High Temperature Superconducting (HTS) compounds produces a stack of weakly coupled xe2x80x98pancakexe2x80x99 vortices whose circulating screening currents are mostly confined within the superconducting CuO2 planes. Superconductors can carry bulk current density only if there is a macroscopic fluxon density gradient. This gradient can be sustained only by pinning the vortices (flux pinning) at microstructural defects. Increasing T and H weaken the potential wells at which vortices are pinned. Flux pinning is determined by spatial perturbations of the free energy of the vortex lines due to local interactions of their normal cores and screening currents with these microstructural imperfections. The critical current density Jc (T, H) is then defined by the balance of the pinning and Lorentz forces. Ideally, a type II superconductor can carry any non-dissipative current density J smaller than Jc. When J exceeds Jc, a superconductor switches into a dissipative, vortex-flow state, driven by the Lorentz force.
High temperature superconducting generators and magnets are significantly lighter and more compact than their conventional counterparts. The development of these devices is of great importance especially in applications requiring compact lightweight, high power sources or compact high field magnets. HTS coated conductor can be used to make the coil windings in HTS generators as well as HTS magnet windings and thus long lengths of coated conductor with high current transport and lower ac losses are desirable.
Various attempts have been made to stabilize or pin the flux vortices within HTS conductors. Introducing impurities or defects into the superconducting material is a known way to provide flux pinning. Such flux pins can be holes, nanotubes, particles, grain boundaries or other defects intentionally introduced into the superconducting material.
Another, recent technique under investigation for introducing flux pinning mechanisms into superconducting materials is to create a coated HTS conductor including a series of layers that alternate between superconducting and non-superconducting materials. See, for example, U.S. Pat. No. 6,191,073 to Hojczyk et al. and U.S. Pat. No. 6,383,989 to Jia et al. (U.S. Patent Application Publication, US 2001/0056041)
While the known methods of improving current carrying capacity within highs temperature superconductors by introducing flux-pinning defects into the superconducting material have achieved some degree of success, they are not without the need for improvement. A need exists for an improved method of manufacturing a high temperature superconductor while concurrently implanting a flux-pinning mechanism therein. Such a method would be relatively simple and inexpensive to implement while providing improved current carrying capability at high temperatures and applied magnetic fields.
Accordingly, it is a primary object of the present invention to provide a method of manufacturing a high temperature superconductor overcoming the limitations and disadvantages of the prior art.
Another object of the present invention is to provide a method of manufacturing a high temperature superconductor providing a multilayered coated conductor having an effective flux pinning mechanism implanted therein.
Yet another object of the present invention is to provide a method of manufacturing a high temperature superconductor that can be readily implemented using known pulsed laser deposition equipment.
Still another object of the present invention is to provide a method of manufacturing a high temperature superconductor that alternately combines a layer of superconducting material with a layer of non-superconducting material, wherein the layer of non-superconducting material is a multiplicity of nanosized globular inclusions of material rather than a uniform layer.
It is still another object of the present invention to provide a method of manufacturing a high temperature superconductor that incorporates alternating layers of superconducting material interspersed with layers of non-superconducting material characterized by a multiplicity of nanosized globular inclusions wherein the material comprising the non-superconducting layer is not chemically reactive with the superconducting material.
It is yet another object of the present invention to provide a method of manufacturing a high temperature superconductor utilizing superconducting YBa2Cu3O7-x (Y123) and non-superconducting Y2BaCuO5-y (Y211).
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, a method of manufacturing a high temperature superconductor utilizing pulsed laser deposition is described. The method of the present invention can be used to fabricate high temperature superconductors using known pulsed laser deposition equipment and techniques.
As can be seen, it is desirable to pin magnetic flux within high temperature superconductors to improve current transport at higher fields. One technique is to add a high density (number) of non-superconducting defects into the superconducting material, for example, xcx9c0.5xc3x971011 M cmxe2x88x922 or xcx9c5 per 100 nm in a linear direction to pin a 5 T field (M being the magnetic field strength in Tesla). This pinning defect should be greater than or equal to the coherence length of the HTS material which, in the case of YBa2Cu3O7-x is in the range of about 1-2 nm. This can become problematic, however, in that many compounds diffuse and react with the HTS material during high temperature processing when the layer thickness is xcx9c1 nm. This leads to degradation of performance and efficiency of the completed HTS conductor.
Advantageously, the method of manufacturing a high temperature superconductor of the present invention utilizes a non-superconducting layer material that is not chemically reactive with the HTS material. This avoids the degradation of performance noted above.
According to the method of the present invention, YBa2Cu3O7-x and Y2BaCuO5-y targets are placed within a pulsed laser deposition chamber. A substrate upon which the superconductor will be grown is also placed within the deposition chamber. The pulsed laser deposition system is placed into operation and a first layer of Y123 is grown upon the substrate by irradiating the YBa2Cu3O7-x target with the emission from the pulsed laser. The process is allowed to continue until a layer of superconducting Y123 is deposited to a thickness of about 7-10 nm. Next, a layer of Y211 is grown upon the layer of Y123 by irradiating the Y2BaCuO5-y target with the emission from the pulsed laser. The process is allowed to continue until a layer of non-superconducting Y211 is deposited to a thickness equal to or exceeding the coherence length of the material, here xcx9c1-2 nm. The layer of Y211 is characterized by a multiplicity of; nanosized globular inclusions, effectively enhancing the flux pinning nature of the Y211 layer. The process is then repeated until a multilayered superconductor is fabricated.